In the discussion of the background that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicants expressly reserve the right to demonstrate that such structures and/or methods do not qualify as prior art.
Detecting and diagnosing subsurface inclusions such as tumors and lumps is a difficult problem. For example, breast or mammary tumors or lumps, or prostate tumors and lumps may be very difficult to detect particularly in the early stages of development. Many imaging devices are available for detecting and diagnosing tumors such as computer tomography (CT), mammogram, ultrasonic imaging, and magnetic resonance imaging (MRI). However, each of these technologies has its disadvantages, for example MRI is relatively expensive and has low specificity; CT and mammogram expose the patient to known dangers of ionized radiation; ultrasound has relatively low contrast, low resolution and low signal to noise ratio.
There are no real good screening tools with high sensitivity and specificity for cancer screening. Sensitivity is defined as the percentage of sick people who are identified as being sick, and specificity is defined as the percentage of healthy people who are identified as being healthy. According to one study, a clinical breast examination has a sensitivity of 54% and a specificity of 94%. See Jatoi 2003. Doppler ultrasound has a sensitivity of 68% and a specificity of 95%. See Raza, et al. 1997. MRI has a sensitivity of 88.1% and a specificity of 67.7%. See Bluemke et al., 2004. Mammography has a sensitivity of 68.6% and a specificity of 94.4%. See Carney, et al. 2003.
Each of the imaging devices mentioned above has difficulty determining how rigid (or elastic) a tumor or lump is. However, there have been studies linking stiffness of the tumor with the cancerous tissues. See Regini, et al. 2010, Wellman, et al. 2001, and Greenleaf, et al. 2003. Moreover, plaque lined blood vessels are less elastic than normal vessels. Thus, stiffness is an important characteristic in determining whether tissue is unhealthy. Furthermore, health care providers often use their finger to probe the tumor or lump to try and determine whether the tumor or lump is rigid; however, the human finger is limited in its ability to determine how rigid a tumor or lump is particularly when the tumor or lump may be relatively deep within the patient's tissue. For example, a lump may be near the chest with several inches of breast tissue between the health care provider's finger and the lump. There is a need for a device that can quantify and display the stiffness of the tumor with greater accuracy than the human finger.
Additionally, when a tumor or lump is detected, the health care provider must determine whether the tumor or lump is benign or cancerous. Currently, an invasive biopsy is typically performed to determine whether or not a tumor is malignant. However, a screening device that can distinguish between a malignant and benign tumor would be extremely useful for early detection. Some progress has been made in determining whether a tumor or lump is benign or cancerous based on properties such as tissue elasticity. See Wilson, et al. 2000, Greenleaf, et al. 2003, and Wellman, et al. 1999. Cancerous lumps tend to be more rigid. More recently, sono-elastography reported a 88.5% sensitivity and 92.7% specificity in diagnosing nodular breast lesions using tissue elasticity. See Regini, et al. 2010. Perhaps more importantly, the same paper noted that out of 120 cases, 116 malignancy cases were correctly identified by the elasticity scores. Thus stiffness of the tumor is a good indicator of the malignancy of the tumor.
There are a few methods for determining the elasticities of tumors. For example, tactile sensors, piezoelectric finger sensor (PEF), and elastography. The artificial tactile sensing methods disclosed in Dargahi, et al. 2004, and Najarian et al. 2009, and the tactile imaging method disclosed in Gaela 2004, use a transduction method to estimate the stress information and obtain elasticity data using a computational model. Commercial palpation imaging systems are available from companies such as Medical Tactile™ Inc. and Assurance Medical™ Inc. These sensors use hundreds of transducer sensors to display the breast examination information. These sensors are used to detect the force applied to the tissue, which is used to calculate the stress. Calculating the stress is necessary to calculate the elasticity of lump or tumor.
Young's modulus (i.e., elasticity) can also be computed from MRI elastography. Also, atomic force microscopy has been used to detect elasticity; however, this is for small local area elasticity measurements. See Vinckier, et al 1998. Both MRI and atomic force microscopy are extremely expensive and cumbersome.
A piezoelectric finger sensor disclosed in U.S. Pat. No. 7,497,133, issued to Shih et al., also can determine the Young's modulus and shear modulus of the contacted objects. A method is disclosed in Shih that uses the piezoelectric finger to apply a force, and then the corresponding displacement is determined by the piezoelectric sensor. However, this method suffers from piezoelectric sensor nonlinearity, hysteresis, and a difficulty in calibrating the piezoelectric finger.
Another approach that detects the elasticity sensations of the organs is elastography. This approach attempts to determine the relative stiffness or elasticity of tissue by using ultrasonic imaging techniques while vibrating the tissue at low frequencies. See Insana, et al. 2004. The research with ultrasound elastography shows that strain information has the potential of distinguishing malignant and benign tumors. See Garra, et. al 1997, Stravos, et al 1993, and Hall 2003. This method includes emitting ultrasonic waves along a path into the tissue and detecting an echo sequence resulting from the ultrasonic wave pulse. However, ultrasound elastography suffers from limited contrast, resolution and low signal to noise problems.
Therefore, there is a need in the art for an apparatus and method for tactile sensor for generating an image of an object.